We here demonstrate the use of multi-walled carbon nanotubes (MWNTs) as a platform matrix for immunoassay using horseradish peroxidase (HRP) as a model protein. After immobilization of HRP onto MWNTs using 1-pyrenebutanoic acid, succinimidyl ester as a cross-linker, the protein-loading capacity of MWNTs was determined by measuring the enzyme activity of immobilized HRP. We also compared the protein binding capacity of MaxiSorp™ – a commercially available polystyrene based microplate – with that of MWNTs. In addition, we assayed the enzyme activity of immobilized HRP in various pH or at high temperature (90°C) in order to examine whether the stability of the immobilized HRP in harsh environments would be changed depending upon the polymer used as a matrix.